1. Field of the Invention
The invention pertains to inertial reference apparatus employing ring laser motion sensors for navigation purposes and more particularly concerns improved double rotation inertial apparatus affording precise determination of craft attitude, position, and velocity in a system relatively less expensive than the conventional gyroscopic inertial platform system.
2. Description of the Prior Art
In the prior art system of the W. G. Wing U.S. Pat. No. 3,563,662 for an "Apparatus for Sensing Movement About a Plurality of Axes", issued Feb. 16, 1971 and assigned to Sperry Rand Corporation, there is presented an inertial reference system in which certain disadvantages of conventional strapped-down inertial references are overcome by mounting ring laser motion sensors, for example three, at respective ones of three orthogonally arranged planes which may be disposed in the form of a portion of a cube. Simultaneous rotation of the three ring-laser sensors to overcome the threshold bias problem explained by Wing and others is provided by rotating the ring laser sensors about a corner-to-corner diagonal of the cube, thereby imparting to each of the ring laser sensors a component of the input angular rate equal to 1/.sqroot. 3 times the input angular rate. Thus, a single rotational input serves to bias all three ring-laser sensors simultaneously above their respective thresholds; a significant mechanical simplification is achieved as compared with a system using individual rotational biasing. The varying frequency outputs of the three ring-laser sensors are fed to a suitable computer and attitude updating computations are performed to maintain, in the computer, quantities which represent the attitude of the effective cube carrying the ring laser sensors with respect to a stable coordinate system. Three acceleration sensors are also mounted at the three planes of the cube with their sensing axes collinear with the sensing axis of the ring laser sensors. The outputs of the accelerometers are also fed to the computer to transform the acceleration information from the coordinates of the cube into the stable coordinate system in order that conventional inertial navigation computations may be performed. It will be appreciated that, in using the above approach, it is not necessary to make an accurate measurement of the rotation rate used for the biasing operation; for the purposes of the navigational computation, the ring laser sensors themselves provide this information. The rate of rotation preferably is high enough to keep all of the ring-laser sensors above their respective thresholds, although it is desirable that the rate be no greater than is required for this purpose because of computational speed considerations and ring-laser scale factor accuracy considerations. Thus, means are provided by Wing whereby the biasing rate input is automatically adjusted to maintain the lowest of the three ring-laser sensor output frequencies always above the threshold value.
The laser ring motion sensor is particularly adaptable to operation in inertial reference systems of the kind described by Wing, as the ring laser sensor is in many respects an equivalent of the conventional mechanical rate gyroscope. However, in addition to the properties of a rate gyroscope, it has inherent characteristics which are uniquely suited to strapped-down inertial navigation systems for use as the angular rate sensors therein. The first of these characteristics is its inherently digital output in that, above a predetermined threshold, it provides an output frequency linearly proportional to its input angular rate. Secondly, it has the ability accurately to measure angular velocity over an extremely wide range and, third, it has a highly accurate scale factor. However, a ring laser sensor has one outstanding deficiency when used in a navigation system in that it has a threshold or dead zone about zero turn rate in which its output frequency is not proportional to its input rate. This is caused by a mode locking phenomenon inherent in ring laser devices. The mode locking phenomenon precludes accurate measurement of low angular rates within the dead zone such as are actually required in navigation systems. Various methods have been suggested for biasing the ring laser sensor at least partially to overcome this deficiency, such as by the use of a Faraday bias cell in one leg of the ring to cause the zero frequency dead zone to occur at some input rate well outside the region of intended operation. One method which does not degrade performance of the ring-laser sensor when utilized in a navigation system is to provide a rotational bias sufficient to maintain the sensor well above its threshold level. A navigation system could be arranged for three-axis sensing in which a trio of ring-laser sensors have their individual sensing axes at right angles with respect to each other and with each rotated about its respective sensing axis at a predetermined rate for biasing purposes. It will be appreciated, however, that such an approach leads to mechanical complexities to be avoided in a strapped-down inertial navigation system.
The inventor Wing fully recognized that large errors could accumulate in his apparatus with respect to use of single axis rotation, a factor which he remedied by periodic reversal of the sense of rotation of the mounting system for his triad of ring-laser sensors. While the Wing apparatus represents an advantageous and useful concept, it demonstrates two problems that are solved by the present inventor. First, during reversals of the Wing apparatus about his single rotation axis, each of the three ring laser sensors goes through the dead zone and useful output information is lost for a significant time period.
The second problem present in the Wing apparatus lies in the effect of the considerable time used in reversing the rotation of the driven system. The Wing apparatus is rigidly connected to a craft that, in the general case, will be undergoing angular rotation about at least one axis. If the Wing inertial measurement device were not experiencing such craft angular rotation, the compensation provided by Wing's reversal techniques would wipe out almost the complete bias. However, in the usual situation, the craft is undergoing angular motion about any of three axis; then, the error vectors do not line up exactly opposite each other and there must be a net angular error, and hence, build-up of navigational errors in the associated navigational computer system. The faster the reversal is effected in the Wing apparatus, the smaller the build-up of navigational error. In actual practice, it is found that rotation reversal in the Wing apparatus would be required in very short periodic intervals, for example, as often as once every thirty seconds. Considering the practical limits imposed upon the design by the inertia of the ring laser sensor system and its supporting structure, that configuration would be almost constantly accelerated and decelerated, requiring the undesired expenditure of considerable power. In such a circumstance, it would also be found that the time period that the ring laser sensors spend in their non-linear or dead zone regions would be quite substantial, further degrading the utility of the information supplied by the inertial measurement unit. It is seen that a relatively higher rate of effective reversal is desired on the foregoing basis. A further advantage of relatively rapid reversal of the system is that the errors associated with low frequency drifts, such as those caused by temperature variation in various parts of each ring-laser sensor, are diminished.